I'd think they'd invite some of the more well known groups out there to come in (or come back) to guest on the show. For example, Rockapella may be nearly unrecognizable nowadays, but they'd be fun to see again. Also some Canadian or international groups (like my local heroes the Heebie Jeebies! Yay!) - also not necessarily to compete, as they'd be slaughtered by any audience voting (Season five of ABDC, I'm looking at you).
I only really know the Footnotes, but that's just form the BOCA angle. Looking forward to this.